The present invention relates to a plasma processing technology, in particular to an RF inductive coupled plasma source with an external discharge bridge.
Plasma is highly ionized gas containing an equal number of positive ions and electrons.
At the present time plasma sources find wide applications in various fields of technology, in particular in the manufacture of semiconductor devices, e.g., for cleaning, etching, deposition, etc., in the production of semiconductor chips.
There exist a great variety of plasma sources which differ in the methods of plasma excitation and the geometry of the electrodes and plasma volume, which, in turn, determine major parameters of the plasma. One type of plasma sources of this type which exists within this variety is known as an inductive coupled plasma. The plasma source of this type is excited by electromagnetic fields having frequencies within the range of few MHz to tens of MHz (RF frequencies).
RF inductive coupled plasma sources (hereinafter referred to as RF ICP sources) find wide industrial application in plasma processing technology. An advantageous feature of RF ICP sources is that electromagnetic energy can be delivered without the requirement for electrodes to be in contact with the plasma. The presence of electrodes can be a serious disadvantage in many applications because the electrodes may generate impurities in plasma volume and transfer them to the final product. The present invention relates specifically to the aforementioned RF ICP source. Such ion sources are described, e.g., by M. A. Lieberman and A. J. Lichtenberg in xe2x80x9cPrinciples of Plasma Discharges and Materials Processingxe2x80x9d, John Wiley and Sons, Inc., N.Y., 1994, p. 387. Typical RF ICP sources of the last generation are described also in: xe2x80x9cIndustrial Plasma Engineeringxe2x80x9d, Vol. 1, Principles, by J. Reece Roth, Institute of Physics Publishing, 1995. As shown in Fig. 11.16, page 413 of this book (see FIG. 1 of the present patent application), an RF ICP source 10 is geometrically similar to DC and RF capacitive hollow cylindrical reactors. A workpiece 12 to be treated, e.g., a semiconductor wafer, is placed into the working chamber 14 of the RF ICP source 10, onto its bottom plate 16. A flat spiral inductive coil 18 which is used for the supply of an RF power from an electric power supply source 17 to the reactor for generation of plasma in the working chamber 14 of the RF ICP source 10 is placed onto the upper surface 20 of the source. The working chamber 14 of RF ICP source 10 contains multipolar permanent magnets (not shown) the purpose of which is to generate a magnetic field around the plasma. This magnetic field improves plasma uniformity, plasma confinement, and increases density of the plasma which in this source may reach 1018 electrons/mxe2x88x923.
A gaseous working medium required for a specific process is fed to the working chamber 14 of the RF ICP source 10 under the pressure of about 1 to 20 mTorr, whereby, at the supply of RF power to the flat spiral inductive coil 18 at a frequency of 1 to 40 MHz, plasma is generated inside the working chamber 14 of the source above and in contact with the workpiece 12. As a result, depending on the type of the working medium, the workpiece is subjected to treatment such as cleaning, etching, coating, etc.
However, for a typical RF ICP source of the type described above, the resonant RF current of the coil 18 should be of about few tens of Amperes, and the RF voltage across the inductor coil 18 reaches few kV. Under such conditions, the RF power loss in a matcher, connectors, and the coil 18 itself, due to the final RF resistance in these elements, is comparable to that transferred to the plasma. Moreover, due to the coil and metal proximity, an essential RF current is induced along the walls of the chamber 14. This means that the power transfer efficiency to the plasma in practical ICP devices is essentially less than 1, and considerable power is dissipated in the ICP hardware.
The ICP source is normally provided with a quartz window. The large RF voltage across the inductor coil 18 (few kV) in ICP sources creates a considerable capacitive current through plasma to the chamber walls and a high DC negative potential on the inner surface of the quartz window. This high DC potential accelerates plasma ions to the window causing its erosion and plasma contamination. Furthermore, the capacitive RF current increases the plasma DC potential to the chamber 14, thus limiting (from the bottom) the minimal energy of ions coming to the substrate 12. This is an undesirable feature too, since energetic ions can damage the substrate 12. Generally, the presence of high RF voltages on the coil 18 and on the matching device causes a lot of problems (such as corona discharge, sparking, and breakdown), and the elimination of these problems takes a significant and costly efforts.
Another common drawback of existing industrial plasma sources using inductive, capacitive and microwave plasma discharge is utilization of relatively high RF frequency (usually 13.56 MHz or higher, up to microwave frequency). This results in expensive, low-efficiency and bulky RF power sources, expensive RF or microwave components in feeders and matching devices, and requires considerable shielding of the RF components bearing high RF voltage and current, to prevent electromagnetic interference with nearby electronic equipment.
An ICP source with reduced frequency and reduced RF voltage was disclosed in pending U.S. patent application Ser. No. 55,452 filed in 1998 by the same applicant. FIG. 2 illustrates a plasma source 36 made in accordance with one embodiment of the aforementioned patent application. This device consists of a sealed working chamber 38, made of such a material as aluminum, fused quartz, ceramic, or the like, which contains an annular transformer magnetic core 40 with a primary winding 42. The core 40 and primary winding 42 are surrounded by a cooling jacket 44 for the supply of a coolant which is fed into the jacket 44 via a coolant inlet pipe 46 and is discharged via a coolant outlet pipe 48. Reference numeral 50 designates a generator. The cooling system protects the core 40 and winding 42 from overheating and thus from variation in the magnetic susceptibility of the ferrite core 40. The plasma is generated in the chamber having a bottom 51 which supports an object to be treated, e.g., a semiconductor wafer 52.
A main disadvantage of such a device is the need in the supply of a cooling agent to the inductor and chamber and problems associated with sealing the coolant from the vacuum. A device made in accordance with still another embodiment of U.S. patent application Ser. No. 55,452 maintains plasma inside a closed metal tube with dielectric seals. The plasma is confined inside the tubes and therefore is applicable only for processes which occur inside the plasma volume, e.g., for plasma synthesis or plasma abatement. Without free plasma surface, for majority of applications in plasma processing of surfaces and in ion sources, the device of this embodiment is not effective.
It is an object of the present invention to provide an inductive RF plasma source with external discharge bridge which is simple in construction, inexpensive to manufacture, is free of the induction of essential RF currents along the walls of the working chamber, has a high power transfer efficiency to the plasma, is free of dissipation of power in the ICP hardware, provides direct contact of plasma surface with the object being treated, prevents damaging of the structural elements of the source with the plasma, provides uniform plasma surface, and makes it possible to increase the efficiency of plasma processing. Another object of the invention is to provide efficient generation of plasma in a wide range of pressures of working medium in the plasma source.